In electronic organs and similar electronic tone producing devices of the key actuated type, it has been the practice to employ circuit arrangements for modifying the decay times of tones upon release of a key. This is done to produce percussion effects or to provide natural sounding tone effects representative of other instruments where the termination of a tone signal is not abrupt, but rather decays in amplitude at a rate determined by the particular effect which is being reproduced by the organ. In the past, it has been common to utilize resistive-capacitive (RC) networks for establishing the desired time delays to create such effects. The capacitors in such timing circuits have relatively large values of capacitance, and it has been necessary to use large numbers of such RC circuits corresponding to the number of keyers used in the instrument. Variations in tolerances of such capacitors make it difficult to consistently reproduce the same effects from one instrument to another. Similar difficulty also is encountered within the same instrument in obtaining uniform decay characteristics among the various keys used in the instrument when RC delay circuits are used.
Another advantage of the use of such RC timing circuits in electronic musical instruments is that at the present state-of-the-art it is not economically feasible to form large value capacitors as part of integrated circuits. Thus, if integrated circuitry is used for other portions of the logic function of such an electronic instrument, it still is necessary to employ discrete capacitors. This requires additional bonding pads on the integrated circuits reducing the usable chip area and also increases manufacturing cost because of the hybrid nature of the circuitry.
Another problem is encountered when it is desired to vary the RC time constants for the purpose of changing the decay envelope of a tone in response to operation of a key activated control switch. Variable capacitors and variable resistors can be employed to permit adjustments of the time delay characteristics of the RC circuits, but this in turn increases the cost of these components. At the same time the consistent reproducability from instrument to instrument and within a given instrument is made increasingly difficult, particularly if reasonable costs in the manufacture of the instrument are to be achieved.
In addition, the use of timing networks tends to introduce unwanted clicks or pops into the reproduced tones whenever the keys are operated to close and open the switches controlled by the key. These audible noises are distracting, and obviously reduce the quality of the music produced by the instrument.
Accordingly, it is desirable to eliminate, to as great an extent as possible, the RC time delay networks in a musical instrument such as an electronic organ and to replace the time delay functions with other types of attenuator networks which do not require large timing capacitors, which are capable of fabrication in integrated circuit form, and the time delay characteristics of which can be readily varied in accordance with the desires of the designer of the instrument and the musician who plays it.